This is a late reply, but may be useful for anyone else struggling with this. This is my solution:

function wpSimpleTestActivate() {
if (version_compare(get_bloginfo('version'), '3.0', '<')) {
trigger_error('You must have at least WordPress 3.0 to use SimpleTest for WordPress', E_USER_ERROR);
}
}

Activation runs in a separate process, so triggering a PHP error will not affect anything else. The error message is shown in the activation status message. It’s shown as a PHP fatal error, so it’s not an elegant solution, but I have not found any other way to return an activation status message.

Also, I’m not sure what Ipstenu is referring too. I took a quick look at WP Super Cache – it’s activation function has absolutely nothing in it.